The invention relates to resins which can be used in can and coil coating compositions.
As is evident from the articles xe2x80x98Pulver-Bandbeschichten bei 60 m/minxe2x80x99 by Dr Graziano (JOT 1996/8, pp. 34-39) and xe2x80x98Polyester based Powder Coatings with reference to Coil Coatingxe2x80x99 by P. Binda (ECCA, Autumn Congress, Brussels, Nov. 18-19, 1996), there is a need for powder paint systems that can be used in coil coating processes at application rates of, for example, 120 m/min. The reactivity of a powder paint composition is too slow for achieving such high rates. The powder paint application techniques (spraying) are also too slow in comparison with the roll application techniques used in can and coil coating processes. Another drawback is that powder paint layers are applied as relatively thick layers. Powder paint resins do moreover generally not comply with foodstuffs legislation. Furthermore, the solid resins having a high glass transition temperature (Tg) used in powder paint compositions are not soluble in organic solvents as used in the coil coating industry.
It is the object of the invention to provide a resin system that can be processed with the aid of the application techniques currently used on an industrial scale in the coil coating industry.
The resin system according to the invention is characterised in that the system comprises a mixture of at least two polymers wherein at least one polymer has a glass transition temperature greater than about 45xc2x0 C. and wherein the polymers are soluble in organic solvents.
Suitable polymers include for example polyesters and polyacrylates.
Preferably the polymers are polyesters.
Preferably, the glass transition temperatures (Tg) of the polymers are different. This difference is generally greater than 5xc2x0 C.
The molecular weights (Mn) of the polymers are usually between about 2000 and about 15000 and preferably they range between about 3000 and about 8000.
Preferably, the resin having a Tg greater than 45xc2x0 C. is amorphous.
Examples of suitable organic solvents which are used for can and coil coating applications include aromatic hydrocarbon resins (for example the xe2x80x98Solvessoxe2x80x99 types), N-methylpyrolidone, xylene, propylene glycol monomethylether, methylpropylene glycol acetate, dibasic ester, isophoron, ethyl ethoxypropionate, ethylene-propylene glycol acetate and/or butyl glycol.
Generally, the second polymer has a dry solids content between about 30% and about 100%. The Tg of the second polymer is generally lower than about 40xc2x0 C.
However it is also possible to apply a second polymer having a Tg higher than about 40xc2x0 C.
Preferably, the second polymer has a dry solids content of at least 50% and a Tg of less than 10xc2x0 C.
The resins may be linear or branched.
The resin system according to the invention can be applied with the present application techniques in can and coil coating processing, because the solid high Tg resins dissolve during the preparation of the paint or varnish and can be applied as solvent borne coating paints. The use of these systems implies also low transport costs and less storage volume before the preparation of the paint.
Coatings with specifically desired properties in the wide application range of both can coatings and coil coatings can be obtained by selecting the appropriate choice of the starting resins in the mixture.
To replace a very wide range of different prior art solvent-bearing resins it is only necessary to make a selection from only a few systems according to the invention because the coating properties can be adjusted by changing the mixing ratio between the resins in the mixture.
Another advantage is the possibility of a flexible choice of solvents because the high Tg resins are soluble in a wide range of solvents.
The weight proportion of the polymer having a Tg higher than 45xc2x0 C. is generally at least 25% and preferably at least 50% (relative to the polymers).
Preferably, the resin system is amorphous because of the desired solubility characteristics.
The polymers are soluble in the organic solvents so that they remain homogeneous and that they show no crystallisation for a period of at least 7 days.
Depending on the desired use, the acid numbers of the polyesters range between about 0 and about 100 mg of KOH/gram of resin and the hydroxyl numbers of the polyesters range between 0 and about 150 mg of KOH/gram of resin.
Systems according to the invention can be used in existing coil coating application lines at rates of up to, for example, 150 m/min and dry layer thicknesses between, for example, 1 and 60 xcexcm.
The polymer mixture according to the invention has to be cured with a crosslinker.
Examples of suitable crosslinkers include compounds containing epoxy groups, compounds containing amino groups and compounds containing isocyanate groups. The crosslinker can be selected depending on the desired use.
Examples of suitable compounds containing epoxy groups are bisphenol A epoxy resins (for example Epikote 828(trademark), Epikote 1001(trademark) and Epikote 1004(trademark) from Shell), hydrogenated bisphenol A epoxy compounds, aliphatic epoxy compounds, epoxidised alkyd resins, epoxidised oils (for example epoxidised linseed oil or soybean oil), epoxidised borates and triglycidyl isocyanurate. Preferably a bisphenol A epoxy resin is used as an epoxy group containing crosslinker.
The carboxyl:epoxy equivalent ratio is generally between 0.85:1 and 1:0.85, preferably between 0.9:1 and 1:0.9.
Examples of suitable amino resin crosslinkers are benzoguanamine, melamine and urea-formaldehyde resins. The polyester:amino resin weight ratio is generally between 95:5 and 60:40 (based on solid resin).
Examples of suitable crosslinkers containing (blocked) isocyanate groups are hexamethylene diisocyanate (HDI), toluene diisocyanate (TDI), isophoron diisocyanate (IPDI), tetramethylxylene diisoycanate (TMXDI), 3,4 isocyanatomethyl-1methyl-cyclohexylisocyanate (IMCI) and their dimers and trimers. Preferably these crosslinkers are blocked.
It is possible to combine the polymers with the same crosslinker. If desired, it is also possible to apply different crosslinkers or mixtures of crosslinkers.
The resins may contain a solid catalyst fused in it. It is also possible to mix a liquid catalyst or a catalyst solution into the paint formulation comprising the resin mixture.
Suitable catalysts for acid-epoxy curing are described by Madec et al. in xe2x80x98Kinetics and Mechanisms of Polyesterificationsxe2x80x99, Advances in Polymer Science, 182-198 (1985). Examples of suitable classes include N-dialkylamine pyridines, tertiary amines, imidazoles, guanidines, cyclic amines and latent amine catalysts. The catalysts can be blocked if so desired.
Examples of suitable catalysts for curing an OH-functional polyester and an amino resin as a crosslinker include strong acids such as sulphonic acids, mono and dialkyl acid phosphate, butyl phosphate and butyl maleate.
Suitable sulphonic acids include for example paratoluene sulphonic acid, methane sulphonic acid, nonyl benzene sulphonic acid, dinonyl naphthalene disulphonic acid and dodecyl sulphonic acid.
Suitable catalysts for curing an OH-functional polyester and an isocyanate based crosslinker include, for example, dibutyl tin dilaureate and zinc octoate.
If catalysts are present, they are generally present in amounts of between about 0.1 and about 5 wt. % (relative to the polyester).
Suitable polyalcohols for preparing the polyesters include ethylene glycol, diethylene glycol, butanediol (1,4), hexanediol (1,6), neopentyl glycol, 2-methyl-1,3-propanediol, 1,3-butanediol, 1,3-propanediol, 1,2-propanediol, 2-ethyl-2-butyl-1,3-propanediol, trimethylpentanediol, hydroxypivalic neopentyl glycol ester, tricyclodecane dimethanol, cyclohexane dimethanol, bisphenol A bishydroxyethyl ether, trimethylolpropane and/or pentaerythritol.
Suitable examples of acids for preparing the polyesters include isophthalic acid, terephthalic acid (dimethyl terephthalate ester), adipic acid, sebacic acid, hexahydroterephthalic acid (CHDA), decane dicarboxylic acid, 5-6-butylisophthalic acid and/or dimerised fatty acids or acid anhydrides such as phthalic anhydride, tetrahydrophthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, pyromellitic anhydride and/or hexahydrophthalic anhydride.
Preferably phthalic anhydride, isophthalic acid, terephthalic acid and/or adipic acid are used.
The esterification reaction preferably takes place under a nitrogen atmosphere at temperatures between 180xc2x0 C. and 260xc2x0 C. Catalysts such as dibutyl tin oxide, tin chloride, butyl chlorotin dihydroxide (FASCAT(trademark)) or tetrabutyoxytitanate and antioxidants such as phosphorous acid, trinonylphenylphosphite or triphenylphosphite can be added as additives. During the reaction the reaction water released is removed through distillation and the desired degree of esterification can be achieved by applying azeotropic distillation and/or vacuum in the last phase.
The reaction results in a polyester that can be dissolved in an amount of organic solvent or in a mixture of organic solvents such that the desired solids content is obtained. The solvent can be added immediately after the polyester synthesis. The solvent is preferably added during the paint preparation.
Suitable solvents include, for example, aromatic hydrocarbon resins (for example Solvesso types), N-methylpyrolidone, xylene, propylene glycol monomethylether, methylpropylene glycol acetate, dibasic ester, isophoron, ethylethoxypropionate, ethylene-propylene glycol acetate and/or butyl glycol. Preferably aromatic hydrocarbons and/or butyl glycol are used.
Coil coatings can be obtained via commonly known processes as described for example in xe2x80x98Coil Coatingsxe2x80x99 by Joseph E. Gaske (Federation of Societies for Coatings Technology, February 1987, pp. 7-19).
The curing conditions and additives can be chosen to depend on the desired peak metal temperature (PMT) and the nature and thickness of the substrate. The curing time will generally be between about 20 and about 70 seconds at temperatures between about 250xc2x0 C. and about 400xc2x0 C. and a PMT between 204xc2x0 C. and 249xc2x0 C.
Suitable substrates include for example steel, tin-plated steel and aluminium.
The coil coatings according to the invention are suitable for use as primer and as top coat and can for example be used as coating for household equipment such as fridges, deepfreezes, microwave ovens, ovens and boilers, as coating for caravans and as coating for facade cladding.
The resin composition according to the invention also yields good results in the can coating industry with which the desired layer thickness generally is thinner and with which the curing conditions differ from the conditions in the preparation of coil coatings.
Can coatings can be obtained via processes of the kind described in for example xe2x80x98Organic Coatingsxe2x80x94Science and Technology, Volume 2: Applications, Properties and Performancexe2x80x99 by Z. W. Wicks et al. (Wiley-Interscience, 1994, pp. 284-290).
The curing conditions and additives can be selected to depend on the desired application and the nature and thickness of the substrate. The curing time will generally lie between a few seconds and tens of minutes at temperatures between about 100xc2x0 C. and about 220xc2x0 C.
Suitable substrates include for example steel, tin-plated steel (ETP, Electrolytic Tin Plate), chromium-plated steel (ECCS, Electrolytic Chromium-Chromium oxide Steel) and aluminium.
The coatings according to the invention are suitable for use as interior and exterior coatings and can be used for example as coatings for beer cans, cans for other beverages (xe2x80x982 and 3 piecexe2x80x99), spray cans, can ends, tubes, drums, cigar boxes and fish cans (the so-called xe2x80x98drawn-redrawn (DRD)xe2x80x99, xe2x80x98draw-wall ironed (DWI)xe2x80x99 cans). They can be used in pigmented or in unpigmented compositions.
The use of the exterior coating is important primarily from a decorative viewpoint, for giving the substrate a saleable appearance. It protects the metal from corrosion and the coating also serves as a label.
The interior coating is mainly intended on the one hand to protect the contents of the can against the influences of the metal and on the other to protect the metal against the contents of the can.
The type of monomers to be used to prepare the polyester, the crosslinkers and the curing conditions can be chosen to depend on the desired use.
The systems according to the invention can be used in pigmented an in unpigmented compositions.
If so desired, the usual additives such as pigments, fillers, stabilisers, dispersing agents, flow-promoting agents and defoaming agents can be added to the binder system according to the invention.